Liquefied natural gas (hereinafter, referred to as LNG) is a colorless and transparent liquid that can be obtained by cooling and liquefying natural gas (predominantly methane) to about −162° C. LNG takes up about 1/600th the volume of natural gas. Therefore, if natural gas is liquefied to LNG, natural gas can be very efficiently transported. For example, an LNG carrier is used to transport LNG by sea.
Since the liquefaction temperature of natural gas is a cryogenic temperature of −163° C. at ambient pressure, LNG is likely to be vaporized even when the temperature of LNG is slightly higher than −163° C. at ambient pressure. In the case of a conventional LNG carrier, even though an LNG cargo tank is thermally insulated, external heat is continuously transferred to LNG. Therefore, during the transportation of LNG by the LNG carrier, LNG is continuously naturally vaporized within the LNG cargo tank and boil-off gas (hereinafter, referred to as BOG) is generated within the LNG cargo tank.
BOG is a type of LNG loss and is an important problem in terms of LNG transportation efficiency. If BOG is accumulated in the LNG cargo tank, a pressure inside the LNG cargo tank is excessively increased. Hence, there is a risk of damage in the LNG cargo tank. Therefore, a variety of methods have been developed to process BOG.
Recently, in order for processing BOG, a method of reliquefying BOG and returning the reliquefied BOG to a cargo tank, a method of using BOG as an energy source for an engine of a vessel, and the like have been used. In addition, a method of burning surplus BOG in a gas combustion unit (GCU) has been used.
When BOG is not useful any more, the GCU inevitably burns surplus BOG so as to adjust the pressure of the cargo tank, resulting in a waste of chemical energy the BOG has.
In a case where a dual fuel (DF) engine is applied as a propulsion system of an LNG carrier, BOG generated in the LNG cargo tank can be processed by using it as a fuel of the DF engine. When an amount of BOG generated in the LNG cargo tank exceeds an amount of fuel used in the DF engine for propulsion of a vessel, BOG may be transferred to the GCU and be burnt for protecting the LNG cargo tank.